In recent years, wireless communication technologies are applied in various fields. For example, in a demodulator of a communication device such as a cellular phone, a wireless LAN (Local Area Network), a radar, a spectrum analyzer, or the like, a receiving device which detects an input signal is used.
For example, in a circuit (receiving device) which demodulates a signal subjected to signal amplitude modulation (AM: Amplitude Modulation), a signal received by an antenna is demodulated by an internal circuit (detector), and the demodulated signal is processed by a voltmeter, an AD (Analog-to-Digital) converter, a digital circuit, or the like. In the receiving device, for example, a range of a proper input signal intensity (input power) for operating the detector is called a dynamic range of the detector.
For example, when the input power is smaller than the lower limit of the usable range of the detector, although the output voltage (detected voltage) of the detector is output, the voltage is too low. Therefore, for example, it is difficult to detect the voltage by a voltmeter at the subsequent stage. When input power is small, for example, an amplifier is provided at the preceding stage of the detector so that the input power becomes larger than the lower limit of the usable range of the detector.
On the other hand, when the input power is larger than the upper limit of the usable range of the detector, an output of the detector is saturated and, for example, it becomes difficult to detect a detected voltage according to the input power by a voltmeter at the subsequent stage. Consequently, when the input power is large, for example, an attenuator (resistor) is provided at the preceding stage of the detector so that the input power becomes smaller than the upper limit of the usable range of the detector.
As described above, the upper and lower limits of the usable range (dynamic range) exist for the input power of the detector. When the input power of the detector lies out of the dynamic range, for example, an amplifier or a resistor is provided at the preceding stage of the detector so that a process of making the input power lie in the dynamic range is performed. In some cases, an amplifier or a resistor may be provided at the subsequent stage of the detector.
However, even when an amplifier or a resistor is provided at the preceding stage (subsequent stage) of the detector, the dynamic range itself is difficult to be enlarged. This is an issue not only for the detector but also for, for example, another internal circuit (receiving device) such as a mixer.
Conventionally, various receiving devices with a characteristic improved by processing an input signal of a detector are being proposed.
Patent Document 1: Japanese Laid-open Patent Publication No. S59-133749
Patent Document 2: Japanese Laid-open Utility Model Publication No. S62-164410
Patent Document 3: Japanese Laid-open Patent Publication No. H02-078306